A typical heating cycle of a hot-water heating system (sometimes referred to as a hydronic heating system) is controlled by a room thermostat. The room thermostat has settings for upper and lower temperature limits. When the indoor air temperature reaches the upper limit, the room thermostat shuts the boiler burner down and when the indoor air temperature reaches the lower limit, it brings the burner on again. The boiler burner is interchangeably referred to herein as a burner or a boiler.
An example of a typical heating cycle includes the room thermostat set for an indoor air temperature of 21° C. The burner cuts in at an indoor air temperature of 20.5° C. and out at 21.5° C. Thus, the indoor air temperature is controlled at 21° C. plus or minus 0.5° C. With this method of working, the burner is on all the time the room thermostat is calling for heat and off when it is not. An example of a winter heating cycle is one that would require the burner to be on for 12 minutes and off for 12 minutes.
Energy losses in a home heating system are due to heat exchange inefficiency and heat-transfer inefficiency. “Heat exchange” as used herein refers to the exchange of heat between the burner and the circulating water. “Heat transfer” as used herein refers to the transfer of heat from the circulating water to the indoor air.
The improvement in heat exchange efficiency afforded by this invention is also applicable to other types of heating systems including forced air heating. However the improvement in heat transfer efficiency also afforded by this invention cannot be applied to heating systems other than hot-water heating systems where the means of heat transfer is not by circulating hot water. With hot-water systems, any heat not transferred from the water to the indoor air is returned to the boiler. With forced air systems, the air passing through the heat exchanger is delivered directly into the living space and therefore not returned via a closed system.
In-situ testing with this invention has shown fuel savings of as much as 50%. Some of these savings are due to improved heat exchange efficiency but a large proportion is due to improved heat transfer efficiency.
Standing losses of energy in the heat exchange process occur during the rest period (burner-off phase) of each cycle when the room thermostat is not calling for heat. When the boiler burner and the circulating water pump are turned off by the room thermostat after satisfying a heat demand, the heat exchanger cools down by dissipating the residual heat in the water and the heat exchanger via the chimney. The dissipating heat, or lost heat, is known as standing losses.
With some boilers the circulating water pump stops at the same time as the burner resulting in none of the standing losses being reclaimed. With other boilers the pump continues running for a short period after the burner has stopped, during which time some of the residual heat in the heat exchanger is reclaimed and passed on to the indoor air.
When the burner comes on at the start of each cycle, the temperatures of both the water and the heat exchanger are initially low. The rates of heat absorbed by the water and the heat exchanger therefore start off high but taper off until maximum temperatures are reached. This phase of the heating cycle is herein referred to as the initial heating period. At this point, second phase heating follows when the temperature of the heat exchanger and the rate of heat absorbed by the water are maintained until the end of the burner-on period when the room thermostat cuts out the burner. A high percentage of heat used during the initial heating period is not used to heat the water.
Home hot-water heating systems are designed and sized to provide adequate heating for the buildings in which they are installed when the outside temperatures are a number of degrees below zero. As a consequence, when the outside temperature is higher than the lowest designed for, the amount of heat contained in the water at the start of circulation is more than that transferred to the indoor air during circulation. The water, therefore, returns to the inlet of the boiler still containing some of the heat it started out with. This does not promote optimum heat transfer. During the burner-on period there can be several circulations like this; the negative effect on heat transfer being cumulative.
The great majority of home hot-water heating systems already in the marketplace suffer from both heat exchange and heat transfer inefficiencies. Thus, an opportunity for efficiency improvements can be attained by addressing these two areas of inefficiency, resulting in energy savings and pollution reduction from reduced burner operation.